A major cause of vibration in a single cylinder engine is piston reciprocation. The piston is started and stopped twice during each rotation of the crankshaft, and reactions to the forces that accelerate and decelerate the piston are imposed upon the engine body as vibration in directions generally parallel to the piston axis. When operated in a device such as a lawn mower, the engine produces vibrations that can be transmitted through the device to the operator. This vibration is uncomfortable and could produce operator fatigue. Even when operated in a device in which there is no issue of operator fatigue (e.g., sump pumps or portable generators), engine vibration is undesirable because it causes maintenance problems and tends to reduce the useful life of the machine.
To some extent such vibrations can be decreased by providing the engine with a counterweight fixed on its crankshaft, and located at the side of the crankshaft axis directly opposite the crankpin by which the piston, through the connecting rod, is connected to the crankshaft. More commonly, two counterweights may be used on the crankshaft, one located on each side of the piston axis. In either case, such a crankshaft counterweight arrangement produces a net resultant centrifugal force vector that is diametrically opposite to the crankpin.
Although such a crankshaft counterweight arrangement can be designed to cancel some or even all of the primary acceleration and deceleration forces on the piston assembly along the piston axis, the centrifugal force of the crankshaft counterweights also has a component transverse to the piston axis. This transverse force component produces lateral vibration, the amount of which increases in direction proportion to the degree to which the crankshaft counterweights successfully cancel out the acceleration and deceleration forces on the piston assembly.
For this reason, most single cylinder engines incorporate crankshaft counterweights having a mass that provides a condition of about “50% overbalance”, such that the centrifugal force due to the counterweights has a component along the piston axis that is equal to about 50% of the acceleration and deceleration forces on the piston assembly. This represents a compromise between the severe vibration in directions parallel to the piston axis that would result with the condition of no overbalance, and the severe vibration transverse to the piston axis that would result with the condition of 100% overbalance.
Because use of crankshaft counterweights having a 50% overbalance condition does not entirely eliminate the undesirable vibration occurring in single cylinder engines, additional techniques have been employed to further reduce such vibration. A number of these techniques employ one or more reciprocating counterweights that, in contrast to the crankshaft counterweights discussed above, do not rotate with the crankshaft but instead “reciprocate” with respect to the crankshaft—that is, move linearly back and forth towards and away from the crankshaft. These reciprocating counterweights are typically coupled to the crankshaft by way of coupling arms, which have near ends coupled to the counterweights and far ends coupled to the crankshaft.
In order that the reciprocating counterweights reciprocate relative to the crankshaft in direct opposition to the reciprocating motion of the piston and crank pin, circular apertures at the far ends of the coupling arms are supported by eccentric journals on the crankshaft. As the crankshaft rotates, the centers of the eccentric journals rotate about the central axis of the crankshaft, and consequently the far ends of the coupling arms also move around the central axis of the crankshaft. Thus, the coupling arms experience a motion that is similar to that of the connecting rod coupling the piston to the crank pin. This connecting rod-type motion, however, is not reciprocating motion since it is not strictly linear motion.
Conventional designs that employ reciprocating counterweights are designed to produce true reciprocating motion of the reciprocating counterweights, so that the motion of the reciprocating counterweights balance the reciprocation of the piston. In order for the reciprocating counterweights to experience linear, reciprocating motion while the coupling arms experience the connecting rod-type motion, the reciprocating counterweights must be rotatably coupled to the coupling arms to allow relative motion therebetween. At the same time, the movement of the reciprocating counterweights must be guided along a linear path, which typically requires that the reciprocating counterweights be additionally coupled to the crankcase.
For example, U.S. Pat. No. 4,656,981 to Murata et al. provides a reciprocating counterweight that is coupled to arms that in turn are supported by eccentric journals on the crankshaft. The reciprocating counterweight further includes a hole in its far end away from the crankshaft. The hole is configured to receive a pin protruding from the crankcase. As the reciprocating counterweight moves, the degree to which the pin extends into the hole varies, and the reciprocating counterweight is thus guided along a linear path defined by the central axis of the pin. The reciprocating counterweight is free to move along the linear path despite the connecting rod-type motion of the coupling arms, since the reciprocating counterweight is rotatably coupled to the coupling arms.
Although these conventional designs are successful to a large degree in balancing the forces of the piston and thus reducing engine vibration, these designs have certain disadvantages. In particular, because the reciprocating counterweight in such a conventional engine must be both rotatably coupled to coupling arms and also coupled to the crankcase to produce true reciprocating motion, the number of parts moving relative to one another and coupling points between these moving parts within the engine is large. The relative motion between the reciprocating counterweight, the coupling arms, and the crankcase can be a source of wear and tear on the engine, and consequently reduce the useful life of the engine. To reduce this wear and tear, the engine further should be designed so that lubrication is provided at the coupling points between the moving parts. Consequently, the costs and complexity associated with designing and manufacturing such an engine is increased.
It would therefore be advantageous if a new balance system for use in single cylinder engines could be designed that was less costly and more robust than conventional balance systems. In particular, it would be advantageous if such a new balance system provided the same or similar benefits of balancing the forces of the reciprocating piston as are provided by conventional designs employing reciprocating counterweights, but at the same time did not require as many parts moving relative to one another as in conventional designs, such that the number of coupling points between those moving parts was reduced, and the need for lubricating multiple coupling points was eliminated.